Floating vessels, such as semi submersibles, drill ships, floating production storage and offloading (FPSO) vessels or the like can be provided with dynamic positioning systems. Such dynamically positioned vessels can use electrically driven propellers, i.e. electric thrusters, to keep position during oil and gas drilling operations, station keeping, anchoring, port maneuvering and the like. For certain types of operations, in which an increased risk of oil pollution, loss of life, collisions or the like exists, it has to be ensured that the position of the vessel is kept for minimizing these risks. Integrity and failure safe operation of the power system which supplies electric power to the thrusters of the dynamically positioned vessel are thus of particular importance.
The vessels can be classified into different classes, such as DP2 (dynamic positioning 2), DP3 or the like. High risk operations such as drilling operations or the approach of other vessels may for example require a certain mode of operation for a particular vessel class. To ensure that a malfunction of a component does not lead to a complete blackout of the power system of the dynamically positioned vessel, the power system needs to be split into several sections, e.g. 2 to 4, in such high risk mode of operation. Each section of the power system is located in a separate engine room, the engine rooms being isolated with fire proof and water tight walls. During such high risk operations, the sections of the power system are electrically isolated, e.g. by opening connections provided by electric cables termed bus ties. One or more engines with connected generators have to be run per power system section in order to supply electric power to connected loads, e.g. to the electric motors of the thrusters. Consequently, in a system with only three sections, three, four or more engines will be running most of the time, the number increasing with the number of sections.
The engines will generally run at relatively low power output, wherein the specific fuel consumption of these engines in the low operating range is generally higher. Consequently, fuel consumption of such power system is high compared to an operational mode in which the sections of the power system can be electrically connected, so that for example only two generators need to be operated, each at a higher load.
Besides the increased fuel consumption and CO2 emission, running several engines in parallel with reduced load can further result in soot accumulating in the combustion chambers, increased operating hours for the generator sets and thus higher costs of maintenance. Since the engines will run most of the time, the blackout risk is also increased.
Operating such system with interconnected power system sections, i.e. with connected bus ties, is generally not possible since a fault, such as a short circuit or generator failure, will generally result in a total blackout of the vessel's power system. Such blackout will result in a loss of position of the vessel, which can be detrimental; it can result in an oil spill or the loss of life. This is for example caused by a propagation of the fault within the power system, so that when bus ties are connected, a fault in one section of the power system will lead to the tripping of components, e.g. generators and thrusters, in other sections of the power system. This can result in the inoperability of most thrusters of the vessel, the vessel thus loosing maneuverability.
It is thus desirable to improve such power systems of dynamically positioned vessel, and to reduce or even eliminate fault propagation in such power systems. It is desirable that most parts of the power system remain operable upon occurrence of a fault. Also, it is desirable to achieve operation with reduced fuel consumption and higher efficiently of the engines powering the generators. It is also desirable to maintain such fuel efficient operation during high risk operations, without compromising the integrity and the safe operation of the power system.